Bullous pemphigoid (BP) is a blistering disease of the skin characterized by development of autoantibodies directed against BP180, a cell-substrate attachment protein. Antibodies of the IgG class have been the focus of most studies of the pathogenesis of this disorder. However, BP patients have an initial urticarial phase of the disease and 70% of these patients have elevated total IgE levels. IgE antibodies that specifically target BP180 have been identified in 90% of BP patients. These IgE autoantibodies predominantly target the same region of the BP antigen as IgG (NC16A), trigger histamine release from basophils and produce lesions that mimic the early stage of lesion development in BP in a xenograft mouse model. A monoclonal antibody that blocks IgE binding to its receptor led to improvement in a BP patient. HYPOTHESIS: Our central hypothesis is that IgE class autoantibodies against BP180 play a critical role in the pathogenesis of BP. More specifically, we propose that BP IgE manifests its effects via multiple mechanisms that involve binding of the IgE either to BP180 on the surface of epidermal keratinocytes or to its high affinity receptor, Fc5RI, on mast cells and potentially eosinophils. These molecular interactions then trigger a pro-inflammatory response in the keratinocytes, eosinophil chemotaxis and degranulation of mast cells. These events will lead to the development of urticarial plaques and subepidermal bullae characteristic of the disease. Specific objectives of the proposal are: Objective 1. Identify the Fc5RI-mediated effects of BP IgE Objective 2. Determine the Fc5RI-independent effects of BP IgE. Objective 3 . Develop a model of IgE autoimmunity utilizing a murine IgE monoclonal antibody directed against human BP180. This is a laboratory based study that will identify the mechanisms by which human BP IgE class autoantibodies are pathogenic. IgE autoantibodies will be isolated from the sera of BP patients and controls by multi-step affinity chromatography. BP IgE will be tested for its ability to trigger mast cell degranulation in the presence of antigen utilizing an RBL-SX cell model. Expression of Fc5RI on circulating and tissue eosinophils in BP will be determined. The ability of BP IgE to cause degranulation, antigen internalization or cytokine production by eosinophils will be tested. Identification of Fc5RI-independent mechanisms will be identified by treatment of human keratinocytes with BP IgE in vitro. The effects of BP IgE on keratinocyte morphology due to binding of the antibody to its antigenic on the cell surface will be determined by light, confocal and electron microscopy. Production of inflammatory mediators by BP IgE-treated keratinocytes will be identified by human cytokine BioPlex and confirmed by ELISA. An IgE monoclonal antibody against NC16A will be tested for its ability to replicate the findings of patient IgE, in order to develop an in-vitro model of IgE autoimmunity. Understanding the role of IgE autoantibodies in BP will provide insight into the role of IgE in immunologic reactions and lead to new areas for therapeutic intervention.